Over the past 50-odd years continuous hot strip mills have grown enormously in length. One of the first such mills, built in 1926 for rolling steel strip, had an overall length of 735 feet from its number 1 furnace to its coiler, and used a typical slab of 4,800 pounds weight. A present day mill spaced for rolling a slab of 2,000 pounds per inch width to a coil would extend about 1,940 feet from its number 1 furnace as to its last down-coiler. A high production mill of this type would have at least four furnaces and the run-out table for those would extend the length of the mill to about 2,240 feet. Depending on slab width, the mill could handle slab and coil weights up to 120,000 pounds for a 60" wide strip.
Mills of this type realize their full efficiencies only when rolling the largest size coils, but seldom roll such coils because of limited slab heating capacity, limitations on the stand motors, and coil handling difficulties in down-stream operation. Most of the time, the mill rolls smaller coils thereby under-utilizing the mill, its driving motors and the space occupied by the entire installation. At their best, continuous hot strip mills are wasteful of space because of the irregular ground area they cover. The mill itself is long and narrow, but the heating furnaces grouped at the entry end require a plot of considerable width, the motor and control room, which is usually on the same side of the mill as the furnaces, requires extra width and encloses much unused space between the widely spaced roughing stands, and the roll shop, usually on the opposite side of the mill from the motor room, should be centrally located with respect to roughing and finishing stands so as to minimize the travel distance through which rolls must be moved between roll stands and the roll shop. Thus, this mill 2,240 feet long, as presently constructed, may require 350,000 square feet of ground area. Only about 40% of the total cost of a continuous hot strip mill is allocated to the mill, electrical apparatus and heating equipment. The remainder goes for buildings, cranes, foundations, utilities and other accessories.
Efforts have been made to alleviate the problem above mentioned by building semi-continuous mills. Instead of having a continuous roughing train of five or six stands with associated edgers, semi-continuous mills normally use one or two roughing stands only, one or both of which are direct current driven reversing stands. The reversing stand or stands reduce the slab in a series of passes before it goes into the finishing train. All stands, of course, are still in line and although there is a substantial saving in overall length of the mill, it is at the expense of its productivity. A variation of this design, sometimes called a three-quarter-mill, uses a roughing train the last two stands of which are close-coupled so that the slab is in both stands for a common unidirectional pass.
Smith U.S. Pat. Nos. 3,803,891 and 3,805,570 disclose another proposed solution. The delay table, so-called, between the roughing and finishing trains is shortened by introducing a coiler and coiling the normally stretched out transfer bar, which is then uncoiled from the coiler into the finishing stands. All stands are still in line.